I did the assistant program in 98/99 in St. quentin en yveslines. On a whole it was a good experience (I met my husband while I was there) I did run in to a few problems. The first being I didn't know if the school would be providing accomodations and how much could I afford since I didn't know how much I would get paid. Coming from Canada, I found it rather difficult trying to find a place to live before I got there. i stayed with the family of one of the teachers for a little while until i found someplace. I found an apt through an ad at the American Church. It was one room on the 6th floor, no elevator, shower in the kitchen and toilet down the hall. All that for 3500 francs (approx 500 euros ) + hydro and phone. (That is a lot since I was only paid 5000 Fr. per month) But it was in the centre of paris and since I worked only 3 days a week, the commute was bearable. Again since I am from canada and on a visa for a specific contract, I could not find other legal work to supplement my income. I was able to work for the fomation continue department in the school which was nice and I got paid alot more. If you are from a EU country, I'm sure you would not have too much trouble finding other work.
Come prepared for your first few classes just in case the teachers do not have any specific direction for you except "Speak English" . that is basically all I got. Otherwise, enjoy the time you have there, visit lots of museums, go out and have fun. If you really want to improve your French, I believe Cretail has a university and if things haven't changed much, you can register for some classes based on your French level. You will have to stand in a LONG line for an oral test. You will get used to LONG lines, just wait until you have to go and get your Carte de Sejour. I realised after a while, exactly why the French always seem to have a book tucked away in a pocket or bag.